Abstract The relationship of electrical conductivity and percolation has been characterised in detail for multiwall carbon nanotubes (MWCNT) in thermoplastic polyurethane (TPU). Weight fractions of MWCNT of up to 5 wt% were studied. Ultrasonic dispersion of the MWCNT during synthesis has been shown to significantly improve homogeneity and percolation of the composite material as evidenced by electron microscopy. Extending ultrasonication time improves the homogeneity of the composite material. The percolation threshold for homogenous MWCNT/TPU material has been determined as 0.95 wt%. Observing surface charging effects with electron microscopy is suggested as a method to probe mesoscopic uniformity. Material uniformity also relates to the magnitude of the apparent conductivity exponent, when conductivity data are fitted with the percolation law. The integration of MWCNT in the polyurethane matrix has been depth-profiled with electron microscopy by varying the energy of the primary electrons. This has shown that many MWCNT curl up into horseshoe shapes. The observation challenges the common assumptions that MWCNT retain their high aspect ratio in the composite material and that they can be modelled as straight rods.

Zone plate design and efficient methods for the fabrication of zone plates for extreme ultraviolet (EUV) and soft x-ray applications in a newly developed scanning reflection microscope are presented. Based on e-beam lithography, three types of transmission zone plates with focal lengths between 6 and 15 mm are reported: (i) phase-shifting zone plates made by 190 nm thick PMMA rings on Si3N4 membranes, (ii) absorbing zone plates made by 75 nm thick Au ring structures on Si3N4, and (iii) freestanding Au rings of 50 nm thickness and increased transmission in the EUV range. Experiments at the DELTA synchrotron facility reveal a minimum spot size and resulting spatial resolution of 9±3  μm, which is the theoretical limit resulting from the synchrotron beam parameters at 60 eV photon energy. Images of a Ti/Si chessboard test pattern are recorded exploiting the energy dependence of the element-specific reflectance.

The mechanical setup of a novel scanning reflection X-ray microscope is presented. It is based on zone plate optics optimized for reflection mode in the EUV spectral range. The microscope can operate at synchrotron radiation beamlines as well as at laboratory-based plasma light sources. In contrast to established X-ray transmission microscopes that use thin foil samples, the new microscope design presented here allows the investigation of any type of bulk materials. Importantly, this permits the investigation of magnetic materials by employing experimental techniques based on X-ray magnetic circular dichroism, X-ray linear magnetic dichroism or the transversal magneto-optical Kerr effect (T-MOKE). The reliable functionality of the new microscope design has been demonstrated by T-MOKE microscopy spectra of Fe/Cr-wedge/Fe trilayer samples. The spectra were recorded at various photon energies across the Fe 3 p edge revealing the orientation of magnetic domains in the sample.

The electronic and topographical structure of vapour-deposited graphene on copper is known to deteriorate in ambient conditions with time. This appears in X-ray absorption spectra at the carbon 1s edge as a reduction of pi*- and sigma*-resonance intensities and as fine structures at energies between the resonances. Our Density Functional Theory calculations show that the intensity reduction is due to the wrinkling of the graphene sheet, which may also cause a hitherto unobserved splitting of the sigma*-resonance. The structure between the resonances can be due to adventitious adsorbates either at grain boundaries or at the graphene surface. The location of adsorbates, such as carboxyl, can be distinguished through the degree of anisotropy of the absorption. The hydrogen and carboxyl adsorbates at the graphene surface correspond to effectively isotropic peaks in the absorption spectrum, since the receiving carbon atom in the graphene sheet adopts sp(3) hybridisation. In contrast, carboxyl groups at the edges of graphene grains are predicted to only cause the anisotropic absorption of photons. This informs the interpretation of an experimentally observed X-ray absorption peak at 288.3 eV, which often persists even after high-temperature vacuum-annealing of graphene and may be caused by adsorbates clustering at the basal plane. (C) 2018 Elsevier Ltd. All rights reserved.

The complex refractive index of many materials is poorly known in the soft X-ray range across absorption edges. This is due to saturation effects that occur there in total-electron-yield and fluorescence-yield spectroscopy and that are strongest at resonance energies. Aiming to obtain reliable optical constants, a procedure that reconciles electron-yield measurements and reflection spectroscopy by correcting these saturation effects is presented. The procedure takes into account the energy- and polarization-dependence of the photon penetration depth as well as the creation efficiency for secondary electrons and their escape length. From corrected electron-yield spectra the absorption constants and the imaginary parts of the refractive index of the material are determined. The real parts of the index are subsequently obtained through a Kramers-Kronig transformation. These preliminary optical constants are refined by simulating reflection spectra and adapting them, so that measured reflection spectra are reproduced best. The efficacy of the new procedure is demonstrated for graphite. The optical constants that have been determined for linearly polarized synchrotron light incident with p- and s-geometry provide a detailed and reliable representation of the complex refractive index of the material near π- and σ-resonances. They are also suitable for allotropes of graphite such as graphene.

The complex refractive index has been determined for graphene for linearly polarised light in p- and s-geometry at energies across the carbon 1s edge. The imaginary part was measured with absorption spectroscopy. The real part was derived using Kramers-Kronig transformations. Results have been validated by X-ray reflectometry of substrate-supported graphene. Theoretical modelling demonstrates that reflection spectra are strongly affected by the relative difference between the reflectance of graphene and its substrate. Measured reflection spectra show that graphene is often sandwiched between a carbonaceous layer of surface-adsorbates and another carbonaceous layer between graphene and substrate. Reflectometry distinguishes adventitious layers from the graphene because of a different energy-dependence of the refractive index. The carbonaceous interface layers can be modelled well with the refractive index function measured for an adventitious surface layer on silicon. The carbon observed at the substrate-interface may originate from the transfer of a graphene sheet from a deposition-substrate. The optically-effective thickness and anisotropy of adventitious carbonaceous layers can be extracted from reflectance spectra. The bonds of adventitious carbon tend to be normal to the basal plane of graphene. Transferred graphene typically has a substrate-interface three times its own thickness and is covered by one layer of surface adsorbates.

Using polarization analysis of linearly polarized synchrotron radiation we demonstrate the existence of a giant magneto-optical Faraday effect at the carbon 1s edge of single-layer graphene on Co, reaching Faraday rotation angles of 2.9 x 10(5)deg/mm. This value is of the order of those observed at the Co 3p and 2p edges. Using element-selective magnetic hysteresis curves we find that graphene on Co exhibits ferromagnetic order. The magnetism in graphene is found to be carried by and be strongly enhanced by aligned n orbitals of carbon atoms. It is induced by hybridization with the Co 3d(z)2 orbitals while carbon a bonds show negligible magnetism due to insignificant hybridization with Co. From additional x-ray magnetic circular dichroism and transversal magneto-optical Kerr effect spectra a magnetic moment of 0.14 mu(B) is estimated for graphene. From Faraday spectra the complete set of x-ray magneto-optical constants of graphene has been deduced which allows for future modeling of magneto-optical devices based on graphene. The strong magnetism in graphene results from hybridization of carbon p(z) and metal 3d orbitals. Atoms of the graphene sublattice A, placed on top of Co, lead to strongest hybridization with Co 3d(z)( )(2)orbitals. Carbon atoms of sublattice B, and those of rotated graphene domains without Co atoms beneath, hybridize with each other and with 3d(xy) and 3d(yz) orbitals of neighboring Co atoms forming tilted p(z) bonds. We show that the related reduction of A-B symmetry leads to a splitting of the spin-polarized density of conduction-band states which is responsible for the strong magneto-optical Faraday effect.

The existence of ferromagnetic ordering in graphene on cobalt is demonstrated by means of resonant magnetic reflection spectroscopy exploiting the transversal magneto-optical Kerr-effect (T-MOKE). Using linearly polarized synchrotron radiation in the soft x-ray range with energies spanning the carbon 1s edge, the π- and σ- bonds of graphene were excited individually, showing that magnetism in graphene is carried by the π – orbitals. Magnetic signals were detected over a wide energy range from 257 – 340 eV with a T-MOKE peak value of 1.1 % at the π – resonance energy near 285 eV. By comparison with corresponding spectra measured at the 2p edges of the Co substrate, a large induced magnetic moment of 0.14 μB was derived for graphene. Individual hysteresis curves monitored at the Co 2p and C 1s edges show that the carbon magnetism is induced by the Co substrate.

Many physical effects are strongly depending on the composition of the interfaces between separating layers. Hence, the knowledge of the interfacial characteristics such as structure, chemical bonds, or magnetic properties of the corresponding materials is essential for an understanding and optimization of these effects. This study reports on a combined magnetic and structural analysis using X-ray photoelectron diffraction (XPD) and transverse magneto-optical Kerr effect (T-MOKE). The information depth of these methods is demonstrated by investigating the uppermost GaAs(001) layer beneath a Fe-film and the interfacial regimes of Fe/GaAs( 001) beneath an MgO capping layer.Iron was prepared on a clean GaAs(001) surface and a GaAs(001)-(4 x 2)-reconstructed surface. Beneath the Fe-film, the (4 x 2)-reconstruction is not lifted, which is clearly shown by the diffraction pattern of the GaAs(4 x 2)-Fe surface. It is shown that Fe inter-diffusion, resulting in an amorphous interface, is almost prevented by the Ga-rich reconstruction. The magneto-optical measurements with T-MOKE clearly demonstrated the Fe-interlayer in a ferromagnetic state. We find no evidence for magnetic properties neither within the signal of the GaAs-substrate nor the MgO-film. (C) 2016 Elsevier B.V. All rights reserved.

Near edge x-ray absorption fine structure spectroscopy was performed to determine the imaginary part of the refractive index of graphite and graphene at wavelengths spanning the carbon K-edge. The real part of the refractive index has been derived from this measured imaginary part via piecewise polynomial Kramers-Kronig transformations. This paper presents the first comparison of simulated reflection spectra using these data with measured reflection spectra.

Graphene deposited by Chemical Vapour Deposition on different copper substrates was characterised using Near Edge X-ray Absorption Fine Structure and X-ray Photoelectron spectroscopies. Pronounced angular dependencies of the carbon pi*- and sigma*-resonances confirmed successful synthesis. Annealing removed surface adsorbates and enhanced the anisotropy of the carbon-bonds, consistent with an observed increase of the carbon 1s binding energy. Most oxygen intercalated at the substrate interface was thus removed, resulting in direct contact between graphene and copper. Oxygen remaining at the interface is bound to carbon. Such bonds are common if graphene on copper is exposed to air for long periods before annealing. The bonds give rise to an X-ray absorption peak at 288.3 eV. When annealed graphene rests in vacuum a deterioration of its structural anisotropy occurs. This is driven by the surface adsorption of oxygen-containing groups. Renewed annealing reverses both effects, which subsequently reoccur. The correlation of anisotropy deterioration and adsorption suggests that by accommodating functional groups at the surface, the hybridization of carbon atoms and the graphene sheet topography are modified. These structural changes occur relatively quickly, so that even in vacuum the graphene sheet only briefly sustains a pristine structure. (C) 2016 Elsevier Ltd. All rights reserved.

The existence of natural birefringence in x-ray reflection on graphene is demonstrated at energies spanning the carbon 1s absorption edge. This new x-ray effect has been discovered with precision measurements of the polarization-plane rotation and the polarization-ellipticity changes that occur upon reflection of linearly polarized synchrotron radiation on monolayer graphene. Extraordinarily large polarization-plane rotations of up to 30 degrees, accompanied by a change from linearly to circularly polarized radiation have been measured for graphene on copper. Graphene on single crystalline cobalt, grown on tungsten, exhibits rotation values of up to 17 degrees. Both graphene systems show resonantly enhanced effects at the pi* and sigma* energies. The results are referenced against those obtained for polycrystalline carbon and highly oriented pyrolytic graphite (HOPG), respectively. As expected, polycrystalline carbon shows negligible rotation, whereas a huge maximum rotation of 140 degrees has been observed for HOPG that may be considered a graphene multilayer system. HOPG is found to exhibit such large rotation values over a broad energy range, even well beyond the pi* resonance energy due to the contributions of numerous graphene layers. To explain the origin of the observed natural birefringence of graphene, the Stokes parameters as well as the x-ray natural linear dichroism in reflection have been determined. It is shown that the birefringence directly results from the optical anisotropy related to the orthogonal alignment of pi* and sigma* bonds in the graphene layer. Our polarization analysis reveals a strong bonding of graphene on Co with a reduced sigma* excitation energy and a strong tilt of 50% of the p(z) orbitals towards diagonal orientation. In contrast, graphene on Cu is weakly bound with an orthogonal orientation of the p(z) orbitals. Exhibiting such a large natural birefringence that can be controlled through substrate choice, and because of excellent heat conductivity, graphene materials have a potential to be used as tunable x-ray phase shifting lambda/4 or lambda/2 plates in the design of future high-intensity light sources.

Reflectivity measurements on graphitic materials such as graphene at energies across the carbon K-edge are frustrated by significant intensity loss due to adventitious carbon on beamline mirrors. Such intensity reduction enhances effects due to perturbing high-order harmonics in the beam. These effects distort the actual structure of the reflectance curve. In order to overcome this limitation, a correction technique has been developed and demonstrated first with measurements for highly ordered pyrolytic graphite. The same approach may be applied to other graphitic materials such as graphene and it may be used with other synchrotron beamlines. The fraction of high-order harmonics was determined by passing the incident beam through a 87nm thin silicon nitride absorber that can be well modeled. Using the corrected measurements the x-ray natural linear dichroism of the sample has been determined.

In graphene oxide, the graphite lattice is intercalated with oxygen groups that bond to carbon atoms. These groups have a bearing on the possibility of using graphene oxide as a precursor to make graphene. The nature of carbon bonds in graphene oxide has been characterized with soft x-ray reflection spectroscopy across the carbon K-edge. Results distinguish graphene oxide synthesized with Hummers' method from that made using a method suggested by Tour. The observed spectra are consistent with those from near-edge x-ray absorption fine structure (NEXAFS) measurements. In particular, the expected carbon K-edge resonances associated with excitations into molecular *- and sigma*-states of CC bonds can be identified. Importantly, the greater oxidation efficiency of the method by Tour may be the reason for the observation of additional resonances that have been assigned to carbon bonding with molecular groups containing oxygen. The additional resonances have been interpreted as the excitations of carbon 1s electrons into the carbonyl *(CO) orbital in the molecular group -COOH and into the hydroxyl *(COH) orbital, respectively. Copyright (c) 2015 John Wiley & Sons, Ltd.

We present combined first-principle calculations and experimental results of the transversal magneto-optical Kerr effect (T-MOKE) of thin Fe films across the 3p edges using linearly polarized synchrotron radiation. We show that the experimental T-MOKE spectra at the 3p edges of Fe exhibit clear signals that are strongly influenced by interference effects. Ab initio calculated T-MOKE asymmetry spectra confirm the importance of interference effects. The comparison of experimental with calculated spectra reveals some differences that we attribute to metal/metal interface roughness that is not taken into account in the calculations.